1. Field of the Invention
This invention relates to a thermal printer operable with differing widths of paper and, more particularly, to such a printer having a platen which is convertible to provide a variable width of rotating platen in accordance with the width of the paper being used.
2. Description of the Related Art
A conventional thermal printer writes on thermally-sensitive paper using a print head having a linear array of heating elements extending across the width of the paper, with the width of the paper being considered to extend in a direction perpendicular to the direction in which the paper is moved through the printer. The paper is held in intimate contact with the print head by means of a platen having an elastomeric outer layer, with the paper moving between the platen and the print head. The platen is rotated by a motor to drive the paper past the print head, with the high coefficient of friction of the elastomer relative to the paper providing a frictional force keeping the paper moving with the surface of the platen. Uniform contact pressure between the paper and the print head is obtained by holding the print head against the paper and the platen with a spring near each end of the print head.
It is desirable to build a thermal printer that can be used efficiently with two different widths of paper. Naturally, the print head and the platen must be wide enough to accommodate the wider paper. A problem associated with such a configuration arises from the fact that, when the narrower paper is used, a substantial portion of the platen rotates against the print head instead of the paper. Depending on the alignment of the narrower paper within the printer, this substantial portion of the platen may extend from one edge of the narrower paper, or this substantial portion may be divided into portions extending from each edge of the narrower paper. In either case, since the paper-moving function of the platen requires the use of an outer layer of the platen having a high coefficient of friction, when the narrower paper is used, a substantial frictional torque is developed on the platen by contact with the stationary print head instead of with the moving paper. This frictional torque increases the torque requirements on the motor driving the printer and may in fact cause this motor to operate at the wrong speed or stall. Also, this frictional torque results in wear of the print head and of the platen.
A solution for this problem is described in Japanese Publication No. 57-74183, which describes a thermal printer platen having a central section turning with a driven shaft and two outer sections, each of which extends from an end of the central section, being rotatably mounted on the shaft. When the wider paper is used, the paper extends across all three platen sections, with the paper being driven by the rotary motion of the central section, and with the outer sections in turn being driven in rotation by the linear motion of the paper. When the narrower paper is used, the paper extends only across the central section, with the paper being driven by the rotary motion of the central section, and with the outer sections remaining stationary against the print head.
Another solution for this problem is described in Japanese Publication No. 57-140177, which describes a platen having end portions coated with a low-friction material. Whether the paper is wide or narrow, it is driven by the conventional central portion. If the paper is narrow, the end portions, having low-friction surfaces, rotate easily against the print head.
Problems with the solutions described in Japanese Publication Nos. 57-74183 and 57-140177 arise from the fact that only the central portion of the wider paper is mechanically driven by the platen. Relatively high levels of friction continue to exist between the outer portions of the wider paper and the print head, so that the margin of safety for driving the paper without jamming is reduced. Furthermore, to avoid an asymmetrical pattern of forces driving the wider paper, the narrower paper is centered within the path provided for the wider paper, while in many applications it is desirable to align a left or right edge of the paper regardless of its width. An example of such applications is found in point of sale printer terminals which prepare sales receipts by cutting a web of paper after printing so that a tab remains at one end before the receipts are separated.
Another solution for the problem of printing on different widths of paper with a thermal printer is described in Japanese Publication No. 6-99621, through the insertion of members composed of material having a low coefficient of friction between the platen and the print head within the regions otherwise occupied by the wider paper when the narrower paper is used. These members may be formed as tabs inserted between the platen and the print head or as belts extending abound the platen and adjacent rollers. What is needed is a method for solving the problem which allows an easier conversion of the apparatus from one paper width to another.
Another solution for this problem is described in Japanese Publication No. 3-150180, with the problem being solved through the variation in the alignment of the print head with the platen. When the wide paper is used, the print head and the platen are aligned with one another. When the narrow paper is being used, the print head and the platen are offset so that they remain in contact through a distance sufficient to accommodate the narrow paper, with opposite end portions of the print head and the platen extending outward past this distance of contact. A disadvantage of this solution is that the required width of the printer is increased by the distance through which the platen extends beyond the print head when the apparatus is configured for printing on the narrow paper.
Yet another solution for this problem is described in Japanese Publication No. 3-27958, which describes a turret having four platens. When a different width of paper is to be used, the turret is rotated to bring a correspondingly long platen into contact with the print head. Disadvantages of this solution arise from the fact that substantial space must be provided within the printer for the turret, that the cost of the printer is increased by the need to provide several platens, and that the rotational drive of the platen must be provided with a capability of engaging and disengaging the various platens as they are brought into place.
Japanese publication No. 4-189-65 describes the use of a pair of spring clutches to actuate the independent rotation of two separate platens on a rotating shaft. These rollers are spread apart, with the spring clutches disposed between them, so that they cannot be used with a single wide paper web, but so that they can readily be used with separate paper webs. A problem with alternatively using such clutches to feed a single wide web of paper with two or more coaxial clutched rollers arises from the fact that such clutches may slip differently, causing the paper to jam as it is fed unevenly.
According to a first aspect of the present invention, there is provided a printing station for printing alternately on a wide paper web and a narrow paper web, in which the printing station includes a split rotary platen, a motor driving a shaft of the split rotary platen, and a print head held against the split rotary platen. The split rotary platen includes the shaft, a main platen roll, attached to turn with the shaft, and a platen roll extension, slidable on the shaft between a first position, in which said platen roll extension engages the main platen roll to turn with the main platen roll, and a second position, in which the platen roll extension is disengaged with the main platen roll and rotatable on the shaft. The split platen roll is longer than a width of the wide paper web. The main platen roll is shorter than the wide paper web, but longer than the narrow paper web.